The present invention relates generally to wireless communication systems and methods of using transmit and receive units with multiple antennas to adapt the transmissions to channel conditions and maximize a communication parameter.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
The wireless communications channels between the transmit and receive devices are inherently variable and thus their quality fluctuates. Hence, their quality parameters also vary in time. Under good conditions wireless channels exhibit good communication parameters, e.g., high signal-to-noise ratio, large data capacity and/or throughput. At these times significant amounts of data can be transmitted via the channel reliably. However, as the channel changes in time, the communication parameters also change. Under altered conditions former data rates, coding techniques and data formats may no longer be feasible. For example, when the channel performance is degraded the transmitted data may experience excessive corruption yielding unacceptable communication parameters. For instance, transmitted data can exhibit excessive bit-error rates or packet error rates. The degradation of the channel can be due to a multitude of factors such as general noise in the channel, multi-path fading, loss of line-of-sight path, excessive Co-Channel Interference (CCI) and other factors.
By reducing CCI the carrier-to-interference (C/I) ratio can be improved and the spectral efficiency increased. Specifically, improved C/I ratio yields higher per link bit rates, enables more aggressive frequency re-use structures and increases the coverage of the system.
It is also known in the communication art that transmit units and receive units equipped with antenna arrays, rather than single antennas, can improve receiver performance. Antenna arrays can both reduce multipath fading of the desired signal and suppress interfering signals or CCI. Such arrays can consequently increase both the range and capacity of wireless systems. This is true for wireless cellular telephone and other mobile systems as well as Fixed Wireless Access (FWA) systems.
In mobile systems, a variety of factors cause signal degradation and corruption. These include interference from other cellular users within or near a given cell. Another source of signal degradation is multipath fading, in which the received amplitude and phase of a signal varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions summed at the antennas of the array.
In FWA systems, e.g., where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move. Still, over time, channel coherence will be lost in FWA systems as well.
Antenna arrays enable the system designer to increase the total received signal power, which makes the extraction of the desired signal easier. Signal recovery techniques using adaptive antenna arrays are described in detail, e.g., in the handbook of Theodore S. Rappaport, Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location; and Paulraj, A. J et al., xe2x80x9cSpace-Time Processing for Wireless Communicationsxe2x80x9d, IEEE Signal Processing Magazine, Nov. 1997, pp. 49-83.
Prior art wireless systems have employed adaptive modulation of the transmitted signals with the use of feedback from the receiver as well as adaptive coding and receiver feedback to adapt data transmission to changing channel conditions. However, effective maximization of channel capacity with multiple transmit and receive antennas is not possible only with adaptive modulation and/or coding.
U.S. Pat. No. 5,592,490 to Barratt et al., U.S. Pat. No. 5,828,658 to Ottersten et al., and U.S. Pat. No. 5,642,353 Roy III, teach about spectrally efficient high capacity wireless communication systems using multiple antennas at the transmitter; here a Base Transceiver Station (BTS) for Space Division Multiple Access (SDMA). In these systems the users or receive units have to be sufficiently separated in space and the BTS uses its transmit antennas to form a beam directed towards each receive unit. The transmitter needs to know the channel state information such as xe2x80x9cspatial signaturesxe2x80x9d prior to transmission in order to form the beams correctly. In this case spatial multiplexing means that data streams are transmitted simultaneously to multiple users who are sufficiently spatially separated.
The disadvantage of the beam-forming method taught by Barratt et al., Ottersten et al., and Roy III is that the users have to be spatially well separated and that their spatial signatures have to be known. Also, the channel information has to be available to the transmit unit ahead of time and the varying channel conditions are not effectively taken into account. Finally, the beams formed transmit only one stream of data to each user and thus do not take full advantage of times when a particular channel may exhibit very good communication parameters and have a higher data capacity for transmitting more data or better signal-to-noise ratio enabling transmission of data formatted with a less robust coding scheme.
U.S. Pat. No. 5,687,194 to Paneth et al. describes a Time Division Multiple Access (TDMA) communication system using multiple antennas for diversity. The proposed system exploits the concept of adaptive transmit power and modulation. The power and modulation levels are selected according to a signal quality indicator fed back to the transmitter.
Addressing the same problems as Paneth et al., U.S. Pat. No. 5,914,946 to Avidor et al. teaches a system with adaptive antenna beams. The beams are adjusted dynamically as the channel changes. Specifically, the beams are adjusted as a function of a received signal indicator in order to maximize signal quality and reduce the system interference.
The last two patents certainly go far in the direction of adaptively changing multiple antenna systems to optimize performance with varying channel conditions. However, further improvements are desirable. In particular, it would be desirable to develop a system where both the transmit unit and receive unit take full advantage of multiple antennas to not only adaptively change the modulation and/or coding but also use a suitable diversity scheme, and spatial multiplexing order all at the same time. These adaptive changes would help to ensure that the communication parameters of the channel remain maximized while the channel varies. Furthermore, it would be an advance in the art to develop a communications system which could take advantage of multiple antennas at the transmit and receive unit to adapt to changing channel conditions and maximize any of a number of desirable communication parameters such as data capacity, signal-to-noise ratio and throughput. This would permit the system to continuously adapt to the type of data being transmitted via the channel.
Accordingly, it is a primary object of the present invention to provide a method to maximize a communication parameter in a channel between a wireless transmit unit and receive unit, both using multiple antennas. Specifically, the method should permit the system to continuously optimize data capacity, signal-to-noise ratio, signal quality, throughput and other desirable parameters while the channel varies.
It is a further object of the invention to provide a method which takes full advantage of multiple antennas at the transmit unit and receive unit to optimize a communication parameter of the channel using a quality parameter derived from the received signals.
Yet another object of the invention is to provide a method as indicated above in any wireless communication system using any combination of multiple access techniques such as TDMA, FDMA, CDMA, and OFDMA.
It is also an object of the invention to provide a wireless communication system taking advantage of adaptive coding, spatial multiplexing, and antenna diversity to continuously maxmize the desired communication parameters under varying channel conditions.
The above objects and advantages, as well as numerous other improvements attained by the method and apparatus of the invention are pointed out below.
The objects and advantages of the invention are achieved by a method of maximizing a communication parameter, such as data capacity, signal quality or throughput of a channel between a transmit unit with M transmit antennas and a receive unit with N receive antennas. The data is first processed to produce parallel spatial-multiplexed streams SMi, where i=1 . . . k. Then, the spatial-multiplexed streams SMi are converted or mapped to transmit signals TSp, where p=1 . . . M, assigned for transmission from the M transmit antennas.
The transmitted signals propagate through the channel and are received in the form of receive signals RSj, where j=1 . . . N, by the N receive antennas of the receiver. The receive signals RSj are used to assess a quality parameter. The quality parameter is used to adaptively adjust k such that the communication parameter of the channel is maximized.
In a preferred embodiment, each of the spatial-multiplexed streams SMi is processed by a coding unit to produce coded streams CSh, where h=1 . . . kxe2x80x2. The quality parameter is utilized in the transmitter to adjust the coding, e.g., by changing kxe2x80x2, used by the coding unit. The coding unit can be a space-time coder, a space-frequency coder, an adaptive modulation rate coder or other suitable coding device. The space-time and space-frequency coders can use different coding and modulation rates.
At the receiver the receive signals RSj are receive processed to reproduce the spatial-multiplexed streams SMi. The quality parameter can be obtained from the receive processed streams SMi. This can be accomplished by a statistical unit which examines streams SMi. In this case the quality parameter can be signal-to-interference ratio, signal-to-noise ratio, power level, level crossing rate, level crossing duration of the signal of a predetermined threshold and reception threshold. Alternatively or in addition the quality parameter can be obtained from reconstituted data. In this case the quality parameter can be the bit-error-rate (BER) or packet error rate.
The mapping step at the transmitter preferably also includes a transmit processing step implemented by a transmit processing block. The quality parameter is then preferably also used for adjusting the processing of the transmit processing block.
Although the quality parameter is typically evaluated at the receiver and fed back or sent to the transmitter in any suitable way, e.g., over a reciprocal channel as used in Time Division Duplexed (TDD) systems, the analysis of the receive signals to derive the quality parameter can be performed by the transmitter. This can be advantageous, e.g., when the receiver does not have sufficient computational resources to derive the quality parameter.
The step of processing the data at the transmitter can be performed by using any suitable coding technique. For example, Space-Time coding or Space-Frequency coding can be used. Meanwhile, the transmit signals TSp are formatted in accordance to at least one multiple access technique such as TDMA, FDMA, CDMA, OFDMA.
The method of the invention can be used between any transmit and receive units including portable and stationary devices. In one embodiment, the method is employed in a wireless network such as a cellular communication system. In this case the method can be used to improve the communication parameter in both downlink and uplink communications.
The method of the invention can be used in existing systems having multiple receive and transmit antennas. The method also permits other useful methods to be employed concurrently. In particular, it is advantageous to use the techniques of the invention together with interference canceling.
A communication system employing the method of the invention achieves adaptive maximization of the communication parameter between its transmit and receive units. The transmit unit has a processing device for processing the data to produce the parallel spatial-multiplexed streams SMi and an antenna mapping device for converting streams SMi to transmit signals TSp and mapping them to the M transmit antennas. The communication system is equipped with a unit for assessing the quality parameter of received signals RSj. In addition, the communication system has a device for adaptively adjusting k based on the quality parameter to maximize the communication parameter. This device can be located in the transmit unit.
The unit for assessing the quality parameter is a statistical unit and is preferably located in the receive unit. Of course, the statistical unit can be located in the transmit unit, as may be advantageous when the receive unit has insufficient resources or power to support the statistical unit.
The communication system also has a coding unit for processing streams SMi to produce coded streams CSh (h=1 . . . kxe2x80x2). The device for adjusting k then also has a mechanism for adjusting kxe2x80x2. The coding unit can be a space-time coder, space-frequency coder or an adaptive modulation and coding rate coder. Preferably, a database of codes and transmit processing parameters is connected to the coding unit and the antenna mapping device.
An adaptive controller is connected to the processing device, the coding unit and the antenna mapping device. The adaptive controller adjusts these based on the quality parameter. Alternatively, the adaptive controller is connected just to the processing device and the antenna mapping device and adjusts them based on the quality parameter.
The communication system can employ any one or more of the available multiple access techniques such as TDMA, FDMA, CDMA, OFDMA. This can be done in a wireless system, e.g., a cellular communication system.